There has been known a vehicle comprising an actuator for changing, in a transmission mechanism which transmits the driving force of an engine to driving wheels via a belt-type continuously variable transmission system, a gear ratio by varying the groove width of a driving pulley. In the normal driving state of the vehicle mentioned above, the groove width of the driving pulley is smaller in the top-ratio range than in the low-ratio range.
In the above-mentioned vehicle, which has a structure in which a centrifugal start-up clutch is provided on the input side (driving side) of the continuously variable transmission system, when the engine is stopped by turning off an ignition switch with the groove width being smaller in the top-ratio range as described above and then the ignition switch is turned on again, a transmission controller gives to the actuator a signal which shifts the gear ratio toward the low-ratio range. As a result, the groove width of the driving pulley becomes larger in the low-ratio range but, since the groove width is enlarged in the state in which a belt is not rotating, the belt may float from the driving pulley in some cases. Even when the engine is started in this state, the belt can remain floating. When a throttle is opened thereafter, the transmission controller generates a transmission instruction signal in accordance with the opening of the throttle so that the groove width of the driving pulley is reduced by the operation of the actuator. There are cases where a smooth start cannot be made because the belt is driven abruptly at the moment at which the driving pulley tucks it in.
To solve the problem described above, a technology is disclosed in Japanese Patent document JP-A No. 249328/1994 which supplies a transmission instruction to the actuator only when the number of revolutions of the engine is over a predetermined allowable number, i.e. only when the belt is rotating.
However, in a hybrid vehicle in which a free wheeling clutch capable of transmitting power in one direction only is provided between a driven pulley and the driving wheels to reduce useless energy consumption by stopping the engine when the opening of the throttle becomes a specified value or less, the number of revolutions of the engine becomes lower than the specified number of revolutions of the start-up clutch immediately after an engine stop instruction so that the driving pulley undesirably stops in an extremely short period of time. This leads to a problem that it has been difficult to reliably return the gear ratio to the low-ratio range by merely setting the allowable number of revolutions for transmission by using the technology of Japanese Patent document JP-A No. 249328/1994 described above, since the time between the stop of the engine and the stop of the belt wound around the driving pulley is extremely short.